1. Field of the Invention
The present invention relates to a vehicle lighting device, and in particular, provides a vehicle lighting device in which a light emitting element is employed as a light source.
2. Description of the Related Art
A light emitting element such as a light emitting diode has a feature that a luminous intensity of light changes depending on a value of a current that is supplied thereto. In view of such a circumstance, a vehicle lighting device in which a light emitting element is employed as a light source is configured in such a manner that the current that is supplied to the light emitting diode is converted to a voltage, the converted voltage is detected, and in accordance with the detected voltage, a substantially constant current is supplied to the light emitting diode.
In this case, it has been known that the light emitting element is configured in such a manner that in consideration of an unexpected decrease of a vehicle power supply (an input power supply), the detected voltage is compared with a value of a voltage that is taken out from a voltage dividing resistor that is connected between the power supply terminal and a ground terminal, and the current that is supplied to the light emitting diode is controlled in accordance with a difference therebetween. FIG. 5 shows the thus configured vehicle lighting device, in other words, a vehicle lighting device in such a manner that a current that is supplied to a light emitting diode 3A is converted to a voltage by means of a voltage detection resistor 2, the converted voltage (the detected voltage) is input to a switching controller 5, and a voltage dividing value of a resistor R21, which can be taken out from a middle connection point in series of a resistor R20 and the resistor R21 that are connected between an input power supply terminal Vin and a ground terminal GND, is input to the reference voltage detecting circuit 5B. A reference voltage detecting circuit 5B is provided with an internal reference voltage in such a manner that, in a case where the voltage dividing value is greater than the internal reference voltage, a constant current is output, or alternatively, in a case where the above value is smaller, an output current is decreased. In this manner, even in a case where a voltage of the input power supply terminal Vin decreases, the current that is supplied to the light emitting diode 3A is decreased.
In addition, it has been known that a vehicle lighting device is configured in such a manner that, in consideration of a change in feature that is exerted by a temperature of a circuit element or the like, the detected voltage is compared with a value of a voltage that is taken out from a voltage dividing resistor made of a thermistor that is connected between a constant voltage source and a ground terminal, and in accordance with a difference therebetween, a current that is supplied to a light emitting diode is controlled. FIG. 6 shows the thus configured vehicle lighting device, in other words, a vehicle lighting device in such a manner the current that is supplied to the light emitting diode 3A is converted to a voltage by means of the voltage detection resistor 2, the converted voltage (the detected voltage) is input to the switching controller 5, and a voltage dividing value, which changes depending on a temperature from a middle connection point in series of the resistor E23 and a thermistor TM3 that are connected to a constant voltage source Vcc and a ground terminal GND, is input to the reference voltage detecting circuit 5B. The reference voltage detecting circuit 5B is divided as described above, and even in a case where an ambient temperature changes, the current that is supplied to the light emitting diode 3A is decreased.
It is to be noted that FIG. 5 and FIG. 6 are views, each of which is drawn in association with FIG. 1 that shows an embodiment of the present invention. Therefore, for a detailed view of another configuration other than those in the foregoing descriptions of FIG. 5 and FIG. 6, reference should be made to a description that is associated with FIG. 1. A technique related to the present invention is found in Japanese Unexamined Patent Application Publication No. 2004-276738, for example. In this publication, there is disclosure of a technique in which a temperature in a vehicle lighting device is detected by means of a temperature sensor such as a thermistor, and when the detected temperature of the temperature sensor exceeds a threshold value, a current of a light emitting element is decreased.
However, the vehicle lighting device that is configured as shown in FIG. 5 has been a device that is capable of carrying out current control due to a decrease of an input power supply, whereas the device is incapable of taking appropriate action such as an action of decreasing a current in a case where an ambient temperature increases; and therefore, the vehicle lighting device of such type has entailed a danger that a circuit or a light emitting diode may break.
Similarly, the vehicle lighting device that is configured as shown in FIG. 6 has been a device that is capable of carrying out current control due to a rise of an ambient temperature, whereas the device is incapable of taking an appropriate action such as decreasing a current in a case where an ambient temperature increases; and therefore, the vehicle lighting device of such type has entailed a danger that a circuit or a light emitting diode may break.
The present invention has been made in view of such a circumstance, and it is an object of the present invention to provide a vehicle lighting device that is configured in such a manner that a current that is supplied to a light emitting element that serves as a light source can be controlled in accordance with either of a voltage change in input power supply and a change in ambient temperature.